Hello all about circuits, I would be very grateful if someone from this fine forum could help me with a problem I have.

I wish to construct a circuit that will switch a 1w LED on for 2 microseconds and then off for 198 microseconds.

This is an example of the kind of LED I would like to use:

http://www.ebay.co.uk/itm/10pcs-Red...al_Components_Supplies_ET&hash=item2a2ea96307

Being a total novice I got as far as researching MOSFETs and programmable resistors before realising this is over my head.

The questions I have are firstly which componants to buy and secondly how to set the circuit to the required timing. I have a friend who can solder that will help me with the practical stuff, but I will need to provide the circuit diagram for us to work from.

Any help from the projects forum will be much appreciated.

 

For such small time delays you would be better with a pic, or some micro controller, arduino.

 

I think you need to back up and define the problem. Do you want one (1) event where the LED is on for 2 microseconds, then off for 198 microseconds and then off for until you tell it to start again? Do you want this to keep repeating? And, why do you need something this fast? Humans can't see changes less than several milliseconds long. Is this for a camera?

 

Thank you for the prompt responses.

#12 I would like the LED to cycle continuously 2 microseconds on and 198 off. I know it's a pretty unusual request; I intend to perform experiment in plant growth (hence 660nm LED)- one that is very likely to fail- but I feel it's worth a try.

Dave- some sort of micro controller would be just the thing, what would you recommend?

 

A 555 timer is hard against the wall at 2 microseconds, so this is going to have to go high-tech. The microprocessor guys will have to take this one.

 

OK so after some quick googling I found this:

http://www.ebay.co.uk/itm/Bid1-iCP1...d=100005&prg=1048&rk=2&rkt=5&sd=221281234450&


seem perfect- plugs into usb, 20mhz oscillator, 1/0 out onboard software doesn't cost much. Could it do the job I want it for though?

 

What do you "think" is going to happen to this plant?
You might has well just leave it in a dark closet.. same experiment.

 

What do you "think" is going to happen to this plant?
You might has well just leave it in a dark closet.. same experiment.

I agree, the most likely outcome is zero growth. However the results of this study:

https://mywebspace.wisc.edu/tsharkey/web/My_papers/Tennessen Bula and Sharkey Pulsed light 1995.pdf suggest otherwise. I would like to repeat this work with a modern high power LED. My interest is in biology rather than electronics hence my post here.

 

Did you read that paper? They're crystal clear that they (and others) didn't see any improvement relative to continuous light.

 

I agree, the most likely outcome is zero growth. However the results of this study:

https://mywebspace.wisc.edu/tsharkey/web/My_papers/Tennessen Bula and Sharkey Pulsed light 1995.pdf suggest otherwise. I would like to repeat this work with a modern high power LED. My interest is in biology rather than electronics hence my post here.

Interesting study..
I didn't read it all but it seems that they are accounting for the decreased "on" time of the light by increasing the PAR well beyond the average delivered to a plant. Is that correct?

 

Did you read that paper? They're crystal clear that they (and others) didn't see any improvement relative to continuous light.

BUT it seemed they also didn't see much of a decrease either as one would "expect" up to a certain point.

It would be interesting to see how the cost of electricity could factor in..

 

Yes, my reading of this paper is that this microsecond cycle fired every chloroplast in the leaf at once rather than having each in a seperate stage of photosynthesis. At the end they refer to LUE (light utilisation efficiency) of 2700% using this method.

It may be I've misread the paper, such a claim seems pretty incredible, but I'm prepared to invest a few quid (<50) and a couple of hours testing it.

 

Well, the plant needs a certain flux of photons to accomplish its biochemical functions. Beyond that, it cannot use more. It reminds me of DNA repair in humans - we can handle the rate of DNA damage due to UV in sunlight but not the rate experienced in a tanning bed.

Anyway, it is a bit surprising (to me, not them) that pulsing at high intensity can supply enough photons even though they are delivered at an "excessive" rate during the brief pulse.

 

It may be I've misread the paper...

I think that's the case! Read it again carefully.

 

Wayne, I am actually satisfied enough with my comprehension of the text to proceed with my project; I do not expect it to work either.

My question was what components would you recommend to create a 2μs on 198μs off cycle in a 1w LED? Which I thought would be relatively easy for a forum as erudite as this.

 

The problem is not the 1% duty cycle, it's delivering 2µs with reasonable accuracy. I believe a rule of thumb is that the clock speed needs to be at least about 10X the pulse, or about 5MHz in your case. That's beyond the capability of the 555 timer widely used around here for such things. You can build oscillators using op-amps, comparators and such but I think it'd be tough.

As noted, you're in the realm of microprocessors. There's a learning curve but you'll be better off in the long run.

If you don't mind spending a bit, you could use something like a LabJack U3 HV (I have one) to output the proper signal, under control of your computer. This has a learning curve as well, but you'd be off and running faster.

 

That labJsack is a bit pricey for my pocket would this piece of equipment not suffice?

OK so after some quick googling I found this:

http://www.ebay.co.uk/itm/Bid1-iCP1...d=100005&prg=1048&rk=2&rkt=5&sd=221281234450&


plugs into usb, 20mhz oscillator, 1/0 out onboard software and it doesn't cost much. Could it do the job I want it for though?

 

I don't know enough to comment, except that it looks very cool for the price. I guess it comes down to how long it'll take to get it running the way you need.

Since the power LED will need a separate power source, you'll need isolation between the power supplies. Hopefully some folks here can help you design a MOSFET switch to operate at that frequency.

 

Thank you Wayne, I can see I may need to re-phrase my question before I can proceed.

Perhaps another of you learned fellows can offer advice, how hard a task have I set myself here- I imagined investing maybe a weekend in the project and spending a few quid on parts, is this completely un-realistic?

 

That's not unrealistic if you can get a schematic to follow. But anything operating in the MHz range is beyond simple DIY projects and requires attention to high frequency design; short wires, minimal inductances and capacitances, and so on. Nothing that isn't done all the time, but more than the typical flashing LED project.

Just to reiterate, you need a 5kHz PWM with a 1% duty cycle. That doesn't sound too hard per se, but if you are doing science then you will need that to be quite accurate. I mean, your numbers will be crap if your 200µs burst is actually 250µs.

If you don't care so much about accuracy, take a look here for an example circuit. See Figure 10.4 at the bottom of the page. I believe that would work for switching a 1W LED at 5kHz. You may replace the 555 and comparator with a signal from your device, but you'll still need the MOSFET and the transistors to drive its gate.